Project Summary Chronic rejection is one of the leading causes of graft loss following transplantation. It is also one of the most significant and unresolved problems requiring investigation and new therapies. The microvascular endothelium is a critical interface between the donor graft and the recipient. Maintaining quiescent and pro-tolerogenic graft endothelium ameliorates chronic rejection. Studies have shown that blockade of the co-inhibitory pathway and their local effects to promote immunoregulation leads to graft rejection. This may be due to the impairment of Foxp3+ regulatory T cell (Tregs)?mediated graft tolerance. My research proposal focuses on improving graft survival and potentially supporting the induction of tolerance within the graft that will benefit all transplant recipients. We found that the endothelial receptor Tie2 and its ligands Angiopoietins (Angpt1/2) maintain the integrity and immunogenicity of endothelial cells. Administration of Angpt1, an activator of Tie2 pathway, and blocking of deactivator ligand Angpt2, (which is otherwise associated with chronic rejection), enhances graft survival and function. We hypothesize that Tie2-mediated signals regulate endothelial PD-L1, and novel PD-L1 agonists will facilitate a tolerogenic intragraft microenvironment that will foster long-term graft survival and local immunoregulation. I will test this hypothesis by two specific aims: 1) To determine Tie2 mediated regulation of PD-L1 and 2) To identify small molecules that regulate PD-L1 expression. We will use endothelial cells that differentially express Tie2. In these studies we will evaluate the Tie2 mediated regulations of different immunoregulatory genes including PD-L1. Luciferase-based assay and high content robotic microscopy workstations will be used to identify PD-L1 agonists in drug libraries. This application for an R03 is intended to allow us to understand the regulation of endothelial PD-L1 and discover novel endothelial PD-L1 agonists by using High Throughput Screening (HTS). At the end of the project, we will be able to identify several agonists of endothelial PD-L1,that will be tested further for biological activity.